This replaces the existing IMount openFor* method with openChannelFor*
ones, which return an appropriate byte channel instead.
As channels are not correctly closed when GCed, we introduce a
FileSystemWrapper. We store a weak reference to this, and when it is
GCed or the file closed, we will remove it from our "open file" set and
ensure any underlying buffers are closed.
While this change may seem a little odd, it does introduce some
benefits:
- We can replace JarMount with a more general FileSystemMount. This
does assume a read-only file system, but could technically be used
for other sources.
- Add support for seekable (binary) handles. We can now look for
instances of SeekableByteChannel and dynamically add it. This works
for all binary filesystem and HTTP streams.
- Rewrite the io library to more accurately emulate PUC Lua's
implementation. We do not correctly implement some elements (most
noticably "*n", but it's a definite improvement.
